1. Field of the Invention
The present invention relates to a display driver circuit having a driver transistor for supplying a current from a power supply to an electroluminescence elements and which controls light emission of the electroluminescence element by controlling the driver transistor.
2. Description of the Related Art
Because electroluminescence (hereinafter simply referred to as “EL”) display devices in which a self-emitting EL element is used as an emissive element in each pixel have advantages such as that the device is thin, self-emitting, and consumes less power, EL display devices have attracted much attention as alternatives to display devices such as liquid crystal display (LCD) and cathode ray tube (CRT) display devices.
In particular, a high resolution display can be achieved by an active matrix EL display device in which a switching element such as a thin film transistor (hereinafter simply referred to as “TFT”) for individually controlling an EL element is provided in each pixel and the EL element in each pixel.
In an active matrix EL display device, a plurality of gate lines extend along a row direction over a substrate, a plurality of data lines and power supply lines extend along a column direction over the substrate, and each pixel has an organic EL element, a selection TFT, a driver TFT, and a storage capacitor. In this structure, a gate line is selected so that the selection TFT is switched on, a data voltage on a data line is charged into the storage capacitor, and the driver TFT is switched on by this data voltage to allow electric power to flow from a power supply line through the organic EL element.
Japanese Patent Laid-Open Publication No. 2001-147659 discloses a circuit in which two p-channel TFTs are added in each pixel as controller transistors and a signal current is applied to a data line.
FIG. 4 shows a pixel circuit disclosed in Japanese Patent Laid-Open Publication No. 2001-147659. As shown, one terminal of an n-channel TFT (selection TFT) 3 having its gate connected to a scan line scanA is connected to a data line data onto which a current Iw is to be applied. The other terminal of the selection TFT 3 is connected to one terminal of a p-channel TFT 1 and one terminal of a p-channel TFT (driver TFT) 4. The other terminal of the TFT 1 is connected to a power supply line Vdd and a gate of the TFT 1 is connected to a gate of a p-channel TFT 2 for driving an organic EL element (“OLED”). The other terminal of the TFT 4 is connected to the gates of the TFTs 1 and 2 and a gate of the TFT 4 is connected to a scan line scanB.
In this structure, the scanA is set to an H level to switch the TFT 3 on and scanB is set to a L level to switch the TFT 4 on. A current Iw corresponding to data is applied to data, which causes the gate and source of the TFT 1 to be short-circuited, the current Iw is converted to a voltage, and voltages of the gates of the TFTs 1 and 2 are set to this voltage. After the TFTs 3 and 4 are switched off, the gate voltage of the TFT 2 is maintained by a capacitor C, thus allowing a current corresponding to the current Iw to flow through the TFT 2 and through an organic EL (OLED) so that light is emitted from the OLED. Then, when scanB is set to an L level, the TFT 1 is switched on, the gate voltage of TFT 1 is increased, the capacitor C is discharged, data is erased, and the TFTs 1 and 2 are switched off.
With this circuit, when a current flows through the TFT 1, the current is converted to a voltage and the gate voltage is determined. According to the determined gate voltage, the amount of current flowing through the TFT 2 is determined. Thus, the amount of current flowing through the TFT 2 can be set corresponding to a signal current Iw.
This circuit, however, requires a scan line scanB for controlling the TFT 4 and the scan line scanB must be driven both when data is written and when data is erased.
In particular, when data is written, both lines scanA and scanB must be driven which results in a heavier load for the driver. In addition, in the erasure process, the TFT 4 is switched on to increase the gate voltage of the TFT 1 , but because the gate voltage is increased via the TFT 1 , there is a problem in that the gate voltage sometimes does not sufficiently increase. In this case, some amount of current continues to flow through the TFT 2 , causing a problem in that black cannot be sufficiently displayed.